This invention relates generally to the field of metallurgy, and more particularly to improved precious metal alloys suitable for casting articles of jewelry, including finger rings, bracelets, earrings and the like. Although certain aspects of the present invention have utility in the casting of non-precious metals, the disclosed technology has particular application in the casting of yellow karat gold alloys in which the percentage of gold is at least 33 percent, e.g. 10 karat.
The casting of articles using such alloys, typically by the so-called "lost wax" process includes problems which are well known in the art, and which have not been readily solved. To reduce labor costs, the cast article should possess a bright outer surface requiring little, if any, further finishing. The mechanical strength of the article is also important, particularly where the article or parts of the same includes parts of relatively thin cross section, because of necessary configuration, or to conserve the use of relatively expensive material. Where improper casting techniques and materials are used, the resultant castings are often of excessively large grain size resulting in correspondingly lower strength, and in some cases, actual cracking in the cast articles. Even in cases where cracks do not initially appear, where, for example, a ring is slightly enlarged, the working of the metal can often result in such cracking. Other problems include excessive hardness of the material, particularly when visible at the exposed surfaces. A particularly common problem is the appearance of "hard spots" of material which project above the finished surface of the article, and which are often so hard and brittle, that they cannot be removed by mechanical operations such as filing and the like. Under certain conditions, the copper content of the alloy provides a blackened oxidized coating on the outer surface of the casting which requires a mechanical and/or chemical operation to remove.
The above problems are not of recent origin, and considerable research has been conducted in the prior art. Some of the problems are solved by removing excess oxygen from the molten alloy, and this has commonly been accomplished by the use of silicon or boron. Unfortunately, such use has undesirable side effects. Silicon is notorious for increasing grain size and porosity, particularly used in the relatively large amounts necessary to achieve effect deoxidization. Boron can be used in relatively lesser amounts, but does produce somewhat similar results. To some extent, these side effects are compensated by the use of other compositions which tend to diminish grain size, such as iridium, nickel, cobalt, and ruthenium. Small amounts of zinc are used to make the alloy somewhat more workable and increase fluidity of the molten alloy when transferred from crucible to flask, and thus improve surface roughness, form filling and strength of the casting. Zinc also has some deoxidizing capability and helps in color shading of yellow gold.
While not commonly used, the use of germanium in amounts of up to one percent of the total volume by weight is not unknown, the germanium serving as a recyclable oxygen scavenger. When used with excessive amounts of boron and silicon, there is a tendency to decolor the yellow appearance of the alloy. When used, it has normally been in combination with lithium, and such use has been confined to gold alloys containing less than 33 percent gold.
To the extent that I have been able to determine, the use of germanium as a sole oxygen scavenging constituent has not been appreciated in the prior art. Yet, in the case of gold karat metal alloys, its use in the absence of silicon and boron enables the use of many known grain enhancement additives in relatively modest amounts to be extremely effective, and without the undesirable characteristics normally present in the cast article.